Cyclohexanol and cyclohexanone, which are the main products of cyclohexane oxidation, are the important raw materials for producing adipic acid and caprolactam. Said oxidation is usually conducted by direct air oxidation (i.e. air is used as an oxidant) in a liquid phase in the presence of a catalyst. In the process of oxidation, a peroxide will form first and then will decompose into some neutral and acidic substances. The acid substances may form esters by reacting with neutral alcohols. Therefore, besides the main products (i.e. cyclohexanone and cyclohexanol), by-products such as dicarboxylic acids, monocarboxylic acids, oxycarboxylic acids and small amount of alcohols, aldehydes, esters and other organic matters with unknown composition also exist in the reaction mixture.
Among these by-products, adipic acid and 6-hydroxycaproic acid have high economic values because they can be converted, by esternfication/hydrogenation, into 1,6-hexanediol, which is an important raw material for producing polyurethane resins and polyester resins.
There have been many patents disclosing the methods of recovering useful organic acids such as adipic acid and 6-hydroxycaproic acid from the reaction mixture of cyclohexane oxidation and converting them into 1,6-hexanediol. For example, JP-B-SHO 53-33567 discloses a method comprising the following steps: basifying the reaction mixture of cyclohexane oxidation with sodium hydroxide; separating the lower layer which is an alkaline solution of sodium salts of organic acids and acidifying it layer with sulfuric acid; separating the obtained oily layer comprising the organic acids from the aqueous layer comprising sodium sulfate; extracting the separated oily layer with an organic substance-free solution of sodium sulfate at a concentration of 15% by weight or higher; combining the obtained aqueous extract and the previously obtained aqueous layer comprising sodium sulfate and again extracting said combined solution with an organic solvent, distilling off the solvents in the organic extract, and utilizing the obtained residues to prepare 1,6-hexanediol through esterification/hydrogenation. Although the method of JP-B-SHO 53-33567 is superior to the methods as described in U.S. Pat. No. 3,524,892 and GP 1,206,417 wherein the substances useful for producing 1.6-hexanediol are recovered by directly extracting the reaction mixture of cyclohexane oxidation with water, it still has the following disadvantages:
1. As the oily layer comprising the organic acids is extracted by an aqueous solution of sodium sulfate, the efficacy for extracting the useful organic acids such as adipic acid and 6-hydroxycaproic acid is difficult to be raised. PA1 2. After extraction by an aqueous solution of sodium sulfate, the waste oily layer, still has higher concentration of sodium ions. When said waste oily layer, alone or together with other fuel oils, is burned in a boiler or a combustion furnace, it will be apt to cause corrosion of the combustion equipment and consequently the lifetime of said equipment may be shortened. PA1 3. The organic solvents for use in extracting the useful organic acids in the aqueous layer are not properly selected. Namely, said method does not use the organic solvents with higher extraction efficacy. PA1 4. When the aqueous extract obtained by extracting the oily layer with the solution of sodium sulfate is combined with the aqueous layer separated from the alkaline solution of sodium salts of organic acids after acidification with sulfuric acid, the organic matters in the combined aqueous solution will increase. If lesser amount of organic solvents are used to extract said combined aqueous solution, the useful organic acids can not be effectively recovered and consequently the residual aqueous solution (also called mother liquor) will have a higher COD value. When the mother liquor is recycled for use, its COD value will increase rapidly. Therefore, upon formation of anhydrous sodium sulfate crystals by concentration, the equipment for concentration and crystallization as well as the quality of sodium sulfate crystals will be adversely affected. In contrast, if more amount of organic solvents are used for extraction, sodium sulfate crystals may be precipitated out and cause problems in handling the extract. PA1 (i)basifying the reaction mixture coming from cyclohexane oxidation with an aqueous solution of an alkali-metal base so that the organic acids essentially comprising adipic acid and 6-hydroxycaproic acid therein, are saponified, extracting the formed alkali metal salts of organic acids from the mixture with water, then acidifying the aqueous extract comprising the alkali metal salts of organic acids to a pH value of 3 or lower with an aqueous solution of a protic inorganic acid, as a result, said aqueous extract is separated into an oily layer and an aqueous layer; PA1 (ii) extracting the organic acid from said oily layer obtained from the step(i) with an aqueous solution of a protic inorganic acid, to obtain an aqueous extract; PA1 (iii) extracting the organic acid from said aqueous layer obtained from the step (i) with an organic solvent selected from alcohols, ketones, esters or the mixtures thereof, to obtain an oily extract; PA1 (iv) extracting the organic acid from the aqueous extract obtained from the step(ii) with an organic solvent selected from alcohols, ketones, esters or mixtures thereof, to obtain an oily extract; and PA1 (v) combining and distilling the oily extracts obtained from the steps (iii) and (iv), to recover the organic acid essentially comprising adipic acid and 6-hydroxycaproic acid.
The present inventor made an effort to resolve the aforesaid disadvantages of the prior art and finally found the method according to the present invention as described below can achieve this purpose. Thus, the present invention has been completed.